Researchers have succeeded in using particles called “muons” to pinpoint the location of nuclear fuel from outside a reactor building in Japan– an advancement that could help locate melted nuclear fuel at the stricken Fukushima No. 1 Nuclear Power Plant.

Muons, elementary particles that come to Earth from space, pass through large rocks and other matter — something X-rays cannot do — but they decay when they hit matter as dense as nuclear fuel.

From February 2012 through December 2013, a research team including members of the High Energy Accelerator Research Organization (KEK) set up and operated measuring equipment at three locations around the Japan Atomic Energy Co.’s Tokai No. 2 Nuclear Power Plant in Ibaraki Prefecture.

They were able to observe muons passing through the reactor building and use the particles to detect the location and size of nuclear fuel stored in a spent-nuclear fuel pool, as well as map the internal structure of the reactor building. The researchers announced their finding on Jan. 23.